NEW MODELS OF MECHANICAL VENTILATION

Introduction

Positive pressure ventilation is described
by trigger, control, and cycle variables.
The trigger initiates a breath. The control
variable remains constant throughout
inspiration regardless of changes in
respiratory system impedance. Inspiration
ends when the cycle is reached. The
relationship between the various possible
breath types and conditional variables is
the mode of ventilation. A mode can include
pressure and volume controlled breaths and
can be sophisticated enough to switch from
one control variable to the other. With each
generation of ventilators, new modes and
other features become available. The purpose
of this paper is to describe the technical
aspects of new modes and related features of
mechanical ventilators that recently have
become available.

The Ventilator Trigger

Patient triggering is usually pressure-tirggered
or flow-triggered. Pressure triggering
requires sufficient patient inspiratory
effort to cause airway pressure to fall from
the set end-expiratory level to a threshold
level (sensitivity) set by the clinician.
With flow triggering, breath initiation is
based on a flow change in the ventilator
circuit beyond some pre-determined
threshold. From the available evidence, the
following recommendations can be made. 1)
The trigger on current generation
ventilators is superior to that which
existed in the past. Auto-triggering can be
problematic secondary to the artifact (e.g.,
cardiac oscillations) when the trigger is
very sensitive. 2) There is no clear
superiority of flow triggering and pressure
triggering. The choice of trigger type
should be based on patient response, using
the trigger type that produces the best
patient comfort. 3) Patient difficulty
triggering the ventilator is usually due to
pathophysiology (e.g., auto-PEEP) rather
than the trigger type or sensitivity.

Pressure Ventilation

Pressure controlled ventilation

With pressure-controlled ventilation (PCV),
the ventilator applies a set pressure to the
airway for a set inspiratory time.
Pressure-controlled breaths can be either
patient-triggered or ventilator-triggered.
Tidal volume during PCV is determined a
number of variables: the pressure control
setting, airways resistance, respiratory
system compliance, auto-PEEP, and patient
effort. Inspiratory time also affects the
tidal volume if the flow does not decrease
to zero. Inspiratory flow is fixed with
volume-controlled ventilation (VCV), whereas
flow with PCV is variable. Because of this,
PCV may be desirable to VCV if the patient
is triggering the ventilator with a strong
respiratory drive. PCV has been advocated by
some authorities as a lung protective
strategy and to improve patient-ventilator
synchrony. However, it has recently been
shown that, with lung protective
ventilation, the work of breathing is
greater with pressure-controlled ventilation
(compared to volume-controlled ventilation).
More important, with pressure controlled
ventilation, volume and trans-pulmonary
pressure limitation is not assured if the
patient makes vigorous inspiratory efforts.

Pressure-controlled inverse-ratio
ventilation

Early reports of improved oxygenation with
pressure-controlled inverse-ratio
ventilation (PCIRV) generated considerable
enthusiasm for this method. Following the
initial enthusiasm for this approach, a
subsequent controlled studies reported no
benefit or marginal benefit for the use of
PCIRV. Based on the available evidence,
there seems to be no clear role for PCIRV in
the management of patients with ARDS. The
likelihood of an improvement in oxygenation
using inverse ratio ventilation is small and
the risk of auto-PEEP and hemodynamic
compromise is great.

Pressure support ventilation

Pressure support ventilation (PSV) assists
inspiratory muscles during invasive and
noninvasive ventilation. PSV is patient
triggered and primarily flow triggered.
Secondary cycling mechanisms with PSV are
pressure and time. Thus, PSV cycles to the
expiratory phase when the flow decelerates
to a ventilator-determined level, when the
pressure rises to a ventilator-determined
level, or the inspiratory time reaches a
ventilator-determined limit. Although PSV is
often considered a simple mode of
ventilation, in reality it can be quite
complex: 1) the ventilator must recognize
the patient’s inspiratory effort, which
depends on the trigger sensitivity of the
ventilator and the presence of auto-PEEP. 2)
The ventilator must deliver an appropriate
flow at the onset of inspiration. A flow
that is too high can produce a pressure
overshoot, whereas a flow that is too low
can produce patient flow starvation and
dyssynchrony. 3) The ventilator must
appropriately cycle to the expiratory phase
without the need for active exhalation by
the patient.

Like PCV, the flow deceleration during PSV
is largely a function of the resistance and
compliance of the respiratory system. The
flow at which the ventilator cycles can
either a fixed absolute flow, a flow based
on the peak inspiratory flow, or a flow
based on peak inspiratory flow and elapsed
inspiratory time. Several studies have
reported dyssynchrony with PSV in subjects
having airflow obstruction (e.g., COPD).
With airflow obstruction, the inspiratory
flow decelerates slowly during PSV, the flow
necessary to cycle may not be reached, and
this stimulates active exhalation to
pressure cycle the breath. This problem
increases with higher levels of PSV and with
higher levels of airflow obstruction.
Several approaches can be used to solve this
problem. 1) PCV can be used, with the
inspiratory time set short enough so that
the patient does not contract the expiratory
muscles to terminate inspiration. 2) On some
newer generation ventilators, the clinician
can adjust the termination flow at which the
ventilator cycles.

The flow at the onset of the inspiratory
phase is determined by rise time - the time
required for the ventilator to reach the PSV
level at the onset of inspiration. Newer
generation of ventilators allow adjustments
of the rise time during PSV. The rise time
is adjusted to patient comfort and
ventilator graphics may be useful to guide
this setting. In patients with a strong
respiratory drive, a rapid rise time may
decrease the work of breathing and the
patient’s sensation of dyspnea. However,
patient comfort may be compromised using
rise times that are either to low or too
high. Moreover, a high inspiratory flow at
the onset of inspiration is not necessarily
beneficial for several reasons. First, if
the flow is higher at the onset of
inspiration, the inspiratory phase may be
prematurely terminated if the ventilator
cycles to the expiratory phase at a flow
that is a fraction of the peak inspiratory
flow. Second, the existence of a
flow-related inspiratory terminating reflex
in the airway has recently been described.
Activation of this reflex due to a higher
inspiratory flow causes shortening of neural
inspiration, which could result in brief,
shallow inspiratory efforts.

Another issue with PSV is the presence of
leaks in the system (e.g., bronchopleural
fistula, cuffless airway, mask leak with
noninvasive ventilation). If the leak
exceeds the termination flow at which the
ventilator cycles, either active exhalation
will occur to terminate inspiration or a
prolonged inspiratory time will be applied.
With a leak, either PCV or a ventilator that
allows an adjustable termination flow should
be used.

Proportional Assist Ventilation

Proportional assist ventilation (PAV) was
designed to increase or decrease airway
pressure in proportion to patient effort,
which should improve patient-ventilator
synchrony. This is accomplished by a
positive feedback control that amplifies
airway pressure proportionally to
inspiratory flow and volume, where
respiratory elastance and resistance are the
feedback signal gains. Unlike other modes of
ventilatory support, which deliver a preset
tidal volume or inspiratory pressure at the
airway, with proportional assist ventilation
the amount of support changes with patient
effort, assisting ventilation with a uniform
proportionality between ventilator and
patient. The advantage of a proportional
ventilatory support lies in its ability to
track changes in ventilatory effort. To the
extent that inspiratory effort is a
reflection of ventilatory demand, this form
of support may result in a more physiologic
breathing pattern.

Tube Compensation

Tube compensation (TC) is designed to
compensate for endotracheal tube resistance
via closed loop control of calculated
tracheal pressure. The proposed advantages
of ATC are to overcome the work-of-breathing
imposed by artificial airways, to improve
patient/ventilator synchrony as a result of
variable inspiratory flow commensurate with
demand, and to reduce air-trapping as a
result of compensation for imposed
expiratory resistance. This system uses the
known resistive coefficients of the tracheal
tube (tracheostomy or endotracheal) and
measurement of instantaneous flow to apply
pressure proportional to resistance
throughout the total respiratory cycle.
Because in vivo tracheal tube resistance
tends to be greater than in vitro
resistance, incomplete compensation for
endotracheal tube resistance may occur.
Additionally, kinks or bends in the tube as
it traverses the upper airway and
accumulation of secretions in the inner
lumen will change the tube’s resistive
coefficient and result in incomplete
compensation.

Whether endotracheal tube resistance poses a
clinical concern for increased
work-of-breathing in adults is
controversial. The imposed work-of-breathing
through the endotracheal tube is modest at
usual minute ventilations for the tube sizes
most commonly used for adults. Several
recent studies cast doubt on the importance
of endotracheal tube resistance during short
trials of spontaneous breathing. For
example, similar outcomes have been reported
when spontaneous breathing trials were
conducted with PSV (7 cm H2O) or
with a T-piece. Moreover, it has been
reported that the work-of-breathing through
the endotracheal tube amounted to only about
10% of the total work-of-breathing. The
work-for-breathing during a two-hour
spontaneous breathing trial with a T-piece
may be similar to the work-of-breathing
immediately following extubation. Although
prolonged spontaneous breathing through an
endotracheal tube is not desirable due to
the resistance of the tube, this may not be
important for short periods of spontaneous
breathing to assess extubation readiness.

Airway pressure-release ventilation

Airway pressure-release ventilation (APRV)
produces alveolar ventilation as an adjunct
to continuous positive airway pressure
(CPAP). Airway pressure is transiently
released to a lower level, after which it is
quickly restored to reinflate the lungs.
Because the patient is allowed to breathe
spontaneously at both levels of CPAP, the
need for sedation is potentially decreased,
hemodynamics are potentially better,
dependent atelectasis may be less, and
oxygenation may be better. Tidal volume for
the APRV breath depends on lung compliance,
airways resistance, the magnitude of the
pressure release, the duration of the
pressure release, and the magnitude of the
patient’s spontaneous breathing efforts. Of
concern is the potential for alveolar
derecruitment during the release of pressure
with APRV.

A modification of APRV is the situation in
which the I:E ratio is not reversed. This is
available on some ventilators as PCV+
(called BIPAP in Europe) or Bilevel. Without
spontaneous breathing, PCV+ is similar to
PCV and APRV is similar to PCIRV. One
potential advantage of these modes is that
the exhalation valve is active during both
the inspiratory and expiratory phase. Prior
to the current generation of ventilators,
the exhalation valve was active during the
expiratory phase, but closed completely
during the inspiratory phase. An active
exhalation valve during the inspiratory
phase will open as necessary to maintain a
constant inspiratory pressure. The use of
APRV has become fashionable in some trauma
centers, but evidence is lacking for
improved patient outcomes compared to
traditional ventilator modes.

One use of PCV+ (or Bilevel) is to provide
sighs during PCV or CPAP. With this
technique, several periods (2 to 4/min) of
elevated airway pressure (25 to 35 cm H2O)
is used periodically as a sigh (1 to 3
seconds at the higher pressure level). This
approach differs from the sighs that were
available in older generation of ventilators
in several ways. 1) They are provided more
frequently. 2) They are pressure limited. 3)
They are applied for a time longer than the
typical inspiratory times set on the
ventilator. 4) Due to the active exhalation
valve, the patient can continue to breathe
spontaneously at the higher pressure.
Although this strategy is attractive in
spontaneously breathing patients prone to
develop atelectasis, its benefit to date is
anecdotal.

Dual Control Modes

Recently developed modes allow the
ventilator to control pressure or volume
based on a feedback loop (dual control). It
is important to appreciate, however, that
the ventilator can only pressure or volume -
not both at the same time. Dual control
within a breath describes a mode where the
ventilator switches from pressure control to
volume control during the breath. Dual
control breath-to-breath is simpler because
the ventilator operates in the either PCV or
PSV, and the pressure limit increases or
decreases to maintain the selected tidal
volume.

Breath-to-breath dual control is available
on several ventilators as Volume Support
(VS). Its proposed advantages are to provide
the positive attributes of PSV with a
constant minute volume. This is closed-loop
control of PSV, wherein tidal volume
provides feedback control for continuously
adjusting the pressure support level. All
breaths are patient triggered, pressure
limited, and flow cycled. The pressure
support level varies breath-to-breath to
maintain a constant tidal volume. The
maximum pressure change is < 3 cm H2O
and can range from 0 cm H2O above
PEEP to 5 cm H2O below the high
pressure alarm setting. Considerable
speculation, but little data, suggests that
VS will wean the patient from pressure
support as patient effort increases and lung
mechanics improve. If the pressure level
increases in an attempt to maintain tidal
volume in the patient with airflow
obstruction, auto-PEEP may result. In cases
of hyperpnea, as patient demand increases,
ventilator support will decrease. This may
be the opposite of the desired response.
Additionally, if the minimum tidal volume
chosen by the clinician exceeds the patient
demand, the patient may remain at that level
of support and weaning may be delayed.

This approach is available as Pressure
Regulated Volume Control (PRVC), Auto-Flow
(Drager Evita 4), and Volume Control Plus
(VC+). This approach provides the positive
attributes of PCV with a constant minute
volume. This mode is a form of pressure
limited, time cycled ventilation that uses
tidal volume as a feedback control for
continuously adjusting the pressure limit.
All breaths are ventilator or patient
triggered, pressure limited, and time
cycled. The pressure increases or decreases
by £ 3 cm H2O per breath to
deliver the desired tidal volume. The
pressure limit fluctuates between PEEP and 5
cm H2O below the upper pressure
alarm setting. The proposed advantage of
PRVC is that it maintains the minimum peak
pressure that provides a constant set tidal
volume and automatic weaning of the pressure
as the patient improves. Perhaps the
greatest advantage of this mode is the
ability of the ventilator to change
inspiratory flow to meet patient demand
while maintaining a constant minute volume.
PRVC and similar modes are attractive with
implementing lung protective strategies
(such as the ARDSnet protocol), because the
tidal volume can me set to 6 mL/kg and the
peak pressure can be set to 30 cm H2O.
However, only anecdotal support of this
approach is currently available.

Conclusions

New ventilator modes and related features
have become available over the past decade,
with the claim that they improve the
efficiency and safety of mechanical
ventilation. Some also claim that these
modes facilitate the weaning process. The
decision to apply a particular mode of
ventilation, however, should also be based
upon an understanding of the underlying
physiology. Just because a new mode does
what it claims does not mean it will be more
useful than existing modes. Unfortunately,
there is very few clinical outcomes data
upon which to base a decision regarding the
choice of ventilator mode. The choice of a
particular mode is often based on clinician
experience and bias, institutional
preferences, and the capabilities of the
ventilators available at that institution.